This invention relates to face type shaft seals and more particularly to face type shaft seals for liquid metal pumps.
Typically primary coolant pumps used in large commercial nuclear power plants have controlled leakage face type seals to retain and confine the radioactive fluid within the pressure containment boundary of the pump. These seals are constructed to operate at moderately high system pressure. This system pressure is therefore utilized as the prime parameter to ensure operational reliability under normal conditions. However, these seals are also required to operate reliably for short periods of time at system pressure conditions far below the normal operating pressure. Prior controlled leakage face type seals are particularly sensitive in this region and are subject to rubbing at low pressure if thermal and/or pressure excursions are experienced.
The face type shaft seal must be capable of operating under normal operating conditions and also at start-up conditions. During normal operating conditions, a liquid film is generally developed between the two seal faces so as to prevent metal-to-metal contact of the two seal faces. However, during initial rotation of the pump shaft, there is generally an insufficient liquid film between the two faces so that a small amount of metal-to-metal rubbing may occur. This is generally not a serious problem because in most seals the metal is of a type that is capable of withstanding slight contact for a minimal amount of time.
In liquid metal pumps developed to date the pump has employed a shaft seal utilizing oil to maintain separation between the two seal faces. In the liquid metal pumps, since the alkali metal coolant being pumped is at a temperature of approximately 400.degree. to 500.degree. C., the pump shaft length must be increased so that the seal can be located a sufficient distance from the heat source so that the seal may be operated without deteriorating due to the extreme temperature of the alkali metal coolant. For example, a typical liquid metal pump shaft is generally more than twice the length of the pump shaft in a typical pressurized water reactor coolant pump. Of course, the increased pump shaft length results in a substantial increase in capital cost for the pump.
In analyzing the liquid metal fast breeder reactor, it becomes apparent that the gas-buffered, oil-lubricated seals of the coolant pump should be replaced by a pump shaft seal capable of operating near the liquid metal coolant. Such a pump shaft seal may be one in which the liquid metal coolant for the nuclear reactor is used as the liquid film between the two faces of the shaft seal. By using such a design, it is possible to greatly reduce the length of the pump shaft and thereby greatly reduce the capital cost of the liquid metal pump. A basic requirement of this type seal, however, is that it be a non-contacting type. This requirement is based on two facts. First, because of compatibility considerations with hot alkali metal, conventional said materials, such as carbon-graphite cannot be considered. As a result, both faces of the seal package must be manufactured from alkali metal compatible metals or ceramics, such as Stellite, carbides, or alumina. None of these materials have self-lubricating, properties, thus introducing a high degree of probability of severe scoring upon initiation of rotation of a contact-type face seal. Secondly, the fluid being sealed, such as sodium, has little if any lubricating ability and is an excellent reducing agent. By the removal of the beneficial boundary lubricant oxide layers, some of the proposed seal materials may quickly possess extremely clean metal surfaces with high self-welding tendencies.
Therefore, what is needed is a face type shaft seal that is capable of being operated in a liquid metal environment.